Traditionally, construction materials have been bonded or secured using materials such as mortar and mechanical fasteners. It is widely acknowledged that these materials have many disadvantages. Specifically, mortar can be slow to set and brittle after curing. Mechanical fasteners are susceptible to corrosion from the elements. In addition, foot traffic on flooring materials often causes the mechanical fasteners to loosen and retract from the materials they are meant to hold in place.
To overcome the disadvantages of traditional bonding or securing methods and materials, various adhesives have been applied in construction applications. However, many of these adhesives have had their own disadvantages. Solvent-based adhesives such as urethanes, latex, acrylics, and bitumens are known in the art but these often have a negative environmental impact. The use of these adhesives has been restricted due to emissions of volatile organic compounds (VOCs). In addition, these adhesives generally have a slow cure rate and can be difficult to handle during application. Adhesives containing chlorinated hydrocarbons or ketone based solvents also have significant disadvantages for use in construction because they can be undesirably odorous, present safety hazards, and are also environmentally unfavorable.
The use of moisture cured, single component polyurethane adhesives to bond various types of construction materials also has been known in the prior art. Generally speaking, a single component polyurethane adhesive refers to a composition of polyurethane chemicals that have been pre-mixed and stored in a dry environment. These single component polyurethane adhesives then react with moisture from the air upon application to a substrate. There are many disadvantages of this single component polyurethane adhesive including slow set time, slow cure rate, high cost per pound, limited container quantities, and poor shelf life.
Some two component polyurethane systems have been used as adhesives in certain applications. However, these two component systems have been limited to use in specific applications and have characteristics which limit their applications further. U.S. Pat. No. 4,244,901 discloses a method of bonding wallboard or drywall to its structural members. The '901 method requires high pressure to apply an expanding polyurethane foam and discloses the use of a "hard" chlorofluorocarbon blowing agent which is not environmentally safe. The high pressure application of this foam, as well as the expansion properties of the foam itself, make this method impractical for application to small area jobs such as installation of ceramic floor tile.
U.S. Pat. No. 4,636,425 discloses a method of bonding with a two component polyurethane system in which a fine mist of a foamable liquid polyurethane is sprayed onto a surface to form discrete droplets or pods of adhesive. This method requires the use of high pressure which also makes the systems impractical for use on a small scale. Additionally, this method discloses the use of a "hard" chlorofluorocarbon, R-11 as the blowing agent.
U.S. Pat. No. 5,362,342 discloses a two component polyurethane adhesive for use in applying roof tiles to a roof substrate. This method discloses application of a stream of adhesive on the roof substrate which expands on the roof substrate. The thickness of this adhesive foam does not significantly diminish after coming into contact with the roof tile which may limit application to other construction materials.
It would be desirable to have an environmentally friendly adhesive and method of bonding that could be used in a broad variety of construction applications and that could be used with low pressure, portable dispensing equipment. It would be particularly desirable if the adhesive could expand to fill voids or imperfectionsbetween the surfaces being bonded, but would then collapse allowing a thin layer of adhesive to bond the construction materials together. Further, it would be desirable if the adhesive would have a rapid cure time and would produce a strong bond that retained some elasticity.